A reader sent in the following question today: “I run on the treadmill in my gym and I’ve noticed that my heart rate creeps up. It’s weird. I warm up, my heart rate gets steady, and then I watch it go up throughout the workout. This seems to be true even if I keep the pace the same. Am I crazy? What’s going on here?”

What you’re describing is actually quite common. The issue is called “heat induced cardiac elevation” or “cardiac drift.” This could be seen equally on either the treadmill or on a stationary bike trainer.

You’ve probably noticed that you’re sweating harder then normal on the treadmill. When running on a treadmill, there is no air moving across your body to cool you down. This means that you have to sweat more to cool yourself. Sweat comes from water in your body that forms a part of the volume of fluids that we refer to as your “blood volume” or “fluid volume”. This is the fluid that your heart pushes through your body, moving oxygen to your muscles and taking waste products away from them.

As you sweat, your blood volume is decreasing due to the fluid that is being lost through you skin to cool you. With less volume of fluid in your body, your heart then has to beat harder to maintain the same workload.

Heat is directly responsible for your heart rate creeping up over time. The way to counter this issue would be to give your body more help in cooling itself: running with a fan pointing at you, wearing very light clothing, putting towels with ice water on your neck and head, and of course, drinking plenty of fluid.

The problem, however, isn’t usually a big one. Although your heart-rate is creeping up, it should only be coming up a small amount – not enough to cause big problems. You can either live with it – now having a better understanding of the issue – or spend more effort cooling yourself to keep your heart rate from rising.

The good news is that you’re not crazy!

Coach Joe

Running Wild with Coach Joe – a blog focused on marathon, triathlon and ultra-endurance racing, training and motivation. Bookmark us at http://coachjoeenglish.wordpress.com or use your favorite RSS feed reader to get the latest news and articles. Running Wild is also available on Yahoo! 360 and My Space.